1. Field of the Invention
The present invention relates generally to an actuator apparatus and method and, more particularly, to a valve actuator including a bonnet assembly having an improved downstop mechanism that is rotatably free with respect to a floating top shaft and engageable with respect to a replaceable operator without affecting bonnet stem drift adjustment.
2. Description of the Related Art
Gate valves are generally comprised of a valve body having a central axis aligned with inlet and outlet passages, and a space between the inlet and outlet passages in which a slide, or gate, may be moved perpendicular to the central axis to open and close the valve. In the closed position, the gate surfaces typically seal against sealing rings which surround the fluid passage through the valve body. Gate valves have been used for centuries to control the flow of a great variety of fluids. Often the fluid to be controlled by the gate valve is under pressure. In the petroleum industry, gate valves are used along piping at various locations, and in particular are used in piping referred to in the petroleum industry as a christmas tree, which is used as part of a drilling operation.
Actuators to open and close the gate valves may include manual operators, diaphragm-type operators, and hydraulic operators. The actuator may include a bonnet assembly, which interconnects the valve body and the valve gate, and a bonnet stem which is movable with the gate via an operator. It is often desirable to be able to change the operator without changing the bonnet assembly. However, this is difficult because, among other reasons, such a change also requires changes in up-stop and down-stop adjustments which assure the drift of the gate is positioned correctly in the open and closed position. If the valve is connected to a christmas tree or is under pressure, it may be difficult to determine whether drift adjustments have been made correctly when replacing the operator since the bore of the valve is not available to receive a drift alignment check tool. Removal of a valve under pressure in a christmas tree to make drift adjustments may take considerable time and cause substantial inconvenience.
It is desirable to combine a manual operator with a diaphragm-type or hydraulic operator for back-up and test purposes. This combination typically results in the presence of a top shaft extending from the operator that may also serve to indicate whether the valve is open or closed. Because the top shaft is often exposed to the atmosphere, it may attract contaminants that cause damage to the top shaft seals or bearings. In the past, close tolerances have been required in the top shaft that have exacerbated the contaminant problems. As well, torque applied to the top shaft, which may be caused by manual operation, may cause gate, gate seal, or drift misalignment. Furthermore, changing the top shaft or the top shaft seals has previously required removal of the operator housing.
The operator typically has a maximum force capability for applying to the bonnet stem. It is sometimes desirable to provide additional opening/closing power on a temporary basis without having to remove the original operator. It is also desirable that the same operator be adaptable to various control accessories, such as a mechanical override, hydraulic override, heat sensitive lock open device, block open cap, electrical limit switch and/or other electrical accessories.
Another significant problem, especially related to diaphragm-type operators, is leakage of the diaphragms in the region adjacent the top shaft or bonnet stem. Such leakage may be caused by wear, loss of flexibility, and pinching or wear that occurs should the diaphragm make contact with the diaphragm case. This leakage may gradually develop, and may slowly reduce the operator power.
In some cases, the positioning of the gate valves in the christmas tree and other types of installations may be restricted because of piping which is supplied to operate an automatic actuator that controls gate movement. In the past, it has been difficult to use precisely laid piping because the position of the operator fluid port is fixed with respect to the operator housing. Allowing the operator to rotate with respect to the bonnet could result in leakage or cause misalignment of the up-stop and downstop drift adjustments of the valve gate.
Thus, there has been a long felt need in the industry to provide an improved actuator that allows a more adaptable installation configuration, that reduces maintenance and installation time, and that increases long term durability. Persons skilled in the art will appreciate the present invention which provides solutions to these and other problems associated with valve actuators.